1. Field of the Invention
The present invention relates to a piezoelectric vibrating reed made of a piezoelectric material, such as quartz and lithium tantalate, a piezoelectric vibrator having the piezoelectric vibrating reed, and an oscillator, an electronic device, and a wave clock each having the piezoelectric vibrator, as well as a manufacturing method of the piezoelectric vibrating reed.
2. Background Art
Recently, a piezoelectric vibrator utilizing quartz or the like is used in a cell-phone and a personal digital assistance as the time source, the timing source of a control signal, a reference signal source, and so forth. The piezoelectric vibrator of this type is proposed in a variety of forms, and a cylinder package type piezoelectric vibrator in which a piezoelectric vibrating reed is encapsulated in a cylindrical case is one example. The piezoelectric vibrator of this type is described, for example, in JP-A-8-298425, JP-A-2001-144581, and JP-A-2001-217677.
As is shown in FIG. 29, a piezoelectric vibrator 200 of this type includes a tuning-fork piezoelectric vibrating reed 201, a case 202 in the shape of a bottomed cylinder to accommodate the piezoelectric vibrating reed 201 inside, and an airtight terminal 203 to hermetically seal the piezoelectric vibrating reed 201 inside the case 202.
As are shown in FIG. 30 and FIG. 31, the piezoelectric vibrating reed 201 is a tuning-fork vibrating reed made of a piezoelectric material of various types and has a pair of vibrating arms 211 disposed parallel to each other and fixed integrally to a base portion 210 on the base end side, a pair of excitation electrodes 212 and 213 formed on the outer surfaces of a pair of the vibrating arms 211 to vibrate a pair of the vibrating arms 211, and a pair of mount electrodes 215 and 216 electrically connected to a pair of the excitation electrodes 212 and 213, respectively, via extraction electrodes 214.
FIG. 30 is a view of the piezoelectric vibrating reed 210 when viewed from above and FIG. 31 is a view of the piezoelectric vibrating reed 210 when viewed from below.
It is configured in such a manner that a voltage is applied to the excitation electrodes 212 and 213 via the mount electrodes 215 and 216, respectively. When a voltage is applied, the excitation electrodes 212 and 213 vibrate a pair of the vibrating arms 211 at a specific resonance in a direction to come close to or move apart from each other. It should be noted that a pair of the mount electrodes 215 and 216 is formed on each of the both surfaces of the piezoelectric vibrating reed 201.
As is shown in FIG. 29, the airtight terminal 203 is formed of an annular stem 220 made of a metal material, two lead terminals 221 provided to penetrate through the stem 220, and a filling material 223 not only to integrally fix the lead terminals 221 and the stem 220 in an insulating state but also to hermetically seal the interior of the case 202.
Regarding the two lead terminals 221, portions protruding inside the case 202 and connected to a pair of the mount electrodes 215 and 216 serve as inner leads 221a and portions protruding to the outside of the case 202 serve as outer leads 221b. The outer leads 221b function as external connection terminals.
The case 202 is press-fit to the outer circumference of the stem 220 and is thereby fit fixedly. Press-fitting of the case 202 is carried out under vacuum atmosphere. Accordingly, a space surrounding the piezoelectric vibrating reed 201 inside the case 202 is hermetically sealed in a state maintained under vacuum.
According to the piezoelectric vibrator 200 configured as above, when a predetermined voltage is applied to the respective outer leads 221b of the two lead terminals 221 as a drive voltage, a current flows into the piezoelectric vibrating reed 201 from the inner leads 221a via the mount electrodes 215 and 216. The piezoelectric vibrating reed 201 thus oscillates at a specific frequency.
The piezoelectric vibrator 200 in the related art, however, has problems as follows.
As is represented by a cell-phone, various electronic devices enclosing the piezoelectric vibrator are becoming smaller in recent years. Accordingly, there is a need for the piezoelectric vibrator to further reduce the size. To date, there is a type having a case diameter D as small as 1.2 mm. However, in order to satisfy the need for a size reduction as above, studies are being conducted for an ultra-compact type having a case diameter D as small as 1.0 mm or further smaller than 1.0 mm. In order to address such a size reduction, a type having a single lead terminal instead of the one having two lead terminals is now proposed.
With a type using two lead terminals as before, even when the diameter of the lead terminals is reduced, the rigidity deteriorates extremely and the lead terminals readily undergo deformation. The lead terminals therefore may no longer be able to ensure electrical independence. To overcome such an inconvenience, a type having a single lead terminal is proposed.
In a case where a single lead terminal is used, the base portion of the piezoelectric vibrating reed is mounted on the lead terminal and the lead terminal is electrically connected to either one mount electrode in a pair of the mount electrodes. In addition, the other mount electrode and the stem of the airtight terminal are directly connected using a wire rod, such as a wire. This configuration makes it possible to electrically connect the other mount electrode and the stem. In other words, in a case where a single lead terminal is used, it becomes possible to vibrate the piezoelectric vibrating reed by applying a predetermined drive voltage to the lead terminal and the stem.
Incidentally, the piezoelectric vibrating reed in the related art has a disadvantage that the R1 characteristic (series resonance resistance value) undesirably becomes higher when configured to correspond to the single lead as described above.
To describe this inconvenience more specifically, as are shown in FIG. 30 and FIG. 31, the excitation electrodes 212 and 213, the extraction electrodes 214, and the mount electrodes 215 and 216 are formed on the both surfaces of the piezoelectric vibrating reed 201. In particular, the mount electrodes 215 and 216 having different polarities are formed on each of the both surfaces of the base portion 210. In other words, the base portion 210 is of a sandwich structure in which it is sandwiched by the mount electrodes 215 and 216 from above and below. This configuration brings the mount electrodes 215 and 216 in a state where they are disposed oppositely with the base portion 210 in between. Accordingly, there arises a problem that the mount electrodes 215 and 216 function as a capacitor and an electrostatic capacity is accumulated between the mount electrodes 215 and 216. This problem gives rise to an inconvenience that the R1 characteristic of the piezoelectric vibrating reed 201 in itself undesirably becomes higher. The vibration performance of the piezoelectric vibrating reed 201 therefore becomes poor and it is difficult to achieve a high-performance piezoelectric vibrator 200. In addition, when the R1 characteristic becomes higher, so does the effective resistance value Re of the piezoelectric vibrating reed 201. It is therefore difficult to make the piezoelectric vibrating reed 201 operate on low power.
The effective resistance value Re is expressed by an equation:Re=[R1(1+(C0/CL)2]where C0 is the original electrostatic capacitance value of the piezoelectric vibrating reed 201 and CL is a constant.